The invention relates generally to the field of media transport and more particularly to print media deskew methods and structural arrangements for use in print applications.
In many print media handling applications, it is desirable to minimize skew, where xe2x80x9cskewxe2x80x9d is defined as the misalignment of print media as a leading edge approaches or reaches a position in which print media orientation affects operations. For applications in which the print media is a sheet of paper or a transparency, the skew will often vary from sheet to sheet. Booklet making is one example of an application in which minimizing skew is an important consideration. U.S. Pat. No. 6,099,225 to Allen et al., which is assigned to the assignee of the present invention, describes what is referred to as a sheet-wise method of booklet making, since the finishing operations are performed on a sheet-by-sheet basis. The finishing operations include cutting, scoring, folding, punching, and stapling. Each sheet is cut to a length that is determined by its sequence in the booklet and by the thickness of the sheets that form the booklet. A sheet that is folded to provide the outer pages of a booklet may not be trimmed at all, while the sheet that is folded to provide the center pages of the booklet will be trimmed by the greatest amount. Because sheets are individually trimmed prior to final assembly, random misalignment of sheets would result in a ragged, unfinished appearance to the booklet. The random skew that is considered to be allowable will vary with the expectations of the manufacturer, but is often a maximum skew that is in the range of one sheet thickness (e.g., xc2x1100 microns) to two sheet thicknesses (e.g., 200 microns).
Buckle deskew mechanisms are used in commercially available printers in which the acceptable skew is much greater. Such buckle deskew mechanisms are not capable of the accuracy required for booklet making. What is needed is a deskew mechanism that is suitable for use in applications in which cost is a significant concern, such as in desktop applications, and/or applications in which precise alignment is a significant concern, such as in sheet-wise booklet making.
The degree of skew of print media progressing along a feedpath to a nip region between deskew rollers is precisely controlled by intentionally introducing print media deformation, with the major directional component of the deformation being perpendicular to the plane defined by the feedpath immediately upstream from the nip region. In one embodiment, mechanical and gravitational forces cooperate to provide the skew control. The print media is driven into the nip region of the deskew rollers in a manner that causes the print media to deform, lifting a movable upper member in a direction substantially perpendicular to the feedpath. However, the upper member applies a return force that is generally equal along a widthwise surface region of the print media, thereby reducing the likelihood that the print media is skewed relative to the nip region.